This invention relates generally to surface mount technology electrical component printed circuit board apparatus and processes and, particularly, to an expanding retainer post for use with printed circuit boards employing surface mount technology and/or with electrical components intended to be attached to a printed circuit board or the like, for example during a surface mount process.
In surface mount technology (sometimes referred to simply as surface mount), electrical components are mounted directly to the surface of a printed circuit board, usually without any contacts or leads penetrating through the printed circuit board. Surface mount technology increases the space utilization efficiency of printed circuit boards and generally reduces the height of components above a board, allowing more printed circuit boards to be stacked or otherwise positioned in a given volume.
In typical surface mount processes, the printed circuit board has circuit traces and contact or mounting pads disposed on one or both of its surfaces. A surface mount electrical component, e.g. a chip carrier, having a plurality of leads or terminal pads, is placed on the board so that the leads or terminal pads touch the contact or mounting pads. Solder has been applied to those leads, terminal pads and/or contact or mounting pads prior to the placement of the components on the board. After all the components are placed on the printed circuit board, thermal energy is applied to "reflow" the solder. Heat may be supplied by an infrared source or by the process known as vapor phase soldering to establish permanent electrical and mechanical connections between the component(s) and the contact or mounting pads and traces on the printed circuit board. Sometimes an adhesive material is used temporarily to hold components in position on the printed circuit board prior to and during the soldering process.
Electrical connectors have been mounted directly to the surface of the printed circuit board in substantially the same way as the other surface mount components are mounted in order to establish all electrical connections at the same time, in the same mounting process. An example of an electrical connector used in surface mount technology is called a header, which is a plurality of electrical contacts retained in a fixed position relative to each other by an engaging, electrically non-conductive body. Header contacts are typically elongated pins having ends that are exposed at opposite sides, e.g., the top and bottom, of the body.
Another method of mounting a connector or other component to a printed circuit board uses retainer posts, usually made of a plastic, that are mounted in holes in the printed circuit board. The retainer posts generally include resilient barbs or other retractable retaining means. The post is inserted into a hole in a printed circuit board under pressure to engage the retaining means in an interference fit with the printed circuit board. The retainer posts can then receive screws, rivets or other fasteners, to hold a connector or other component to the printed circuit board.
If conventional printed circuit retainer posts are used in surface mount technology processing, they can be damaged by the temperatures used in the soldering step. In the typical vapor phase soldering step, temperatures can reach 400.degree. to 450.degree. F. (220.degree. to 235.degree. C.), causing shrinkage of the retainer posts and printed circuit board. That is, a printed circuit board hole increases in size, resulting in a loose fit between the board and the retainer post. This loose grip means that items, such as connectors, attached to the retainer posts can move relative to, and in the plane of, the printed circuit board threatening the integrity of electrical connections to surface mounted components and reliability. The loose mechanical connection can also make the printed circuit board susceptible to incorrect insertion in a connector. Nevertheless, retainer posts have considerable utility since they permit mechanical mounting of a wide variety of items on a printed circuit board.
Accordingly, it is desirable to provide a retainer post for a printed circuit board that is suitable for use with surface mount component technology, and that can withstand the temperature of vapor phase soldering without the loss of, and preferably with an improvement in, the security of the mechanical connection between the retainer post and the printed circuit board in which the post is installed.